Recently, a need for a semiconductor laser, which can provide not only low-noise characteristics at light output power of 3 mW to 5 mW but also a high output power of 20 mW or larger (in general, 30 mW to 40 mW), has increased as light sources for reading and writing in mini disks and optical magnetic disks.
Initial characteristics which are required for this type of semiconductor laser include (1) single transverse mode oscillation up to high output power operation, (2) low astigmatism, (3) low ellipticity, and (4) low noise.
Of these characteristics, in order to satisfy (1) the single transverse mode oscillation up to high output power operation (that is, a phenomenon in which light output power is not monotonously increased in accordance with an increase of a current up to a desired high output power operation but the output power lowers at a certain current (hereinafter referred to as "kink") is not generated) and (2) low astigmatism, techniques to provide low-reflectance coating to the facet of an outgoing beam side as well as to employ an index waveguide structure as a semiconductor laser chip structure are adopted. This is because in the index waveguide structure, a single transverse mode and reduced astigmatism are easy to obtain. In addition, if the outgoing beam side facet is designed to be a low reflectance, a beam output efficiency is increased and in addition to further increase the current level that generates the kink, reliability can be improved. In order to achieve low ellipticity, each current injection width of chips is narrowed to increase the beam divergence angle parallel to the active layer or the active layer is made with thin films so that the beam divergence angle vertical to the active layer is made smaller. All these techniques intend to further increase an index difference in the lateral direction (parallel to the active layer) and do not interfere the above-mentioned required characteristics (1), (2) (for example, see pages 50 to 60 or pages 108 to 110, "Semiconductor Laser" by Ryoichi Ito et al, Baifukan 1989).
However, a laser having an index waveguide structure generally causes single longitudinal mode oscillation even in the low output power operation (for example, in the vicinity of beam output power 3 mW) when information is read from disks such as photo magnetic disks and results in high coherence. The semiconductor laser with high coherence as described above is assumed to be easy to generate induced noise caused by feedback light, and is unable to satisfy low noise (4) described above. That is, the laser beam reflected and returned from the disk plate impinging in the semiconductor laser again causes the laser oscillation to be unstable.
To cope with this, conventionally, a method has been employed, in which high-frequency current of several hundreds of mega hertz (MHz) is superimposed over the laser driving current to multiplex the longitudinal mode so that coherence is lowered, thereby reducing optical feedback noise (Ryoichi Ito et al, "Semiconductor Laser," Baifukan, 1989, pages 314 to 315, as described above). However, this method required a high-frequency superimposing circuit in addition to a regular semiconductor laser driving circuit, creating problems of increased package size, high consumption current, and high costs. In addition, because high-frequency electromagnetic noise is generated, it causes serious problems in handling electronic equipment such as computers.
On the other hand, as methods to lower coherence without using the high-frequency superimposing technique, there are methods to (a) use a gain waveguide structure and (b) generate self-oscillation. However, but method (a) has a problem which results in increased astigmatism in addition to tendency to cause an unstable transverse mode. In the case of method (b), it is difficult to stably obtain the single transverse mode up to a high output power operation and it is easy to cause the kink, and it is also practically impossible to obtain light output power exceeding 20 mW.
That is, the conventional semiconductor laser which can provide high output power for writing has a problem of large noise at low output power for reading and the conventional semiconductor laser which can provide low noise characteristics at low output power for reading is unable to provide high output power for writing.